1. Field of the Invention
This invention relates to an optical pickup device for recording and/or reproducing signals on or from an optical disk.
2. Related Prior Art
There is so far known an optical pickup device which is provided with a semiconductor laser unit as a light source and in which a light beam emanating from this semiconductor laser unit is converged by a suitable optical system on a signal recording surface of an optical disk to write and/or read data signals on or from the signal recording surface. Examples of this type of the optical pickup device is disclosed in U.S. Pat. Nos. 4,766,583 and 4,751,694. The optical pickup device disclosed therein is provided with a photosensor, such as a photodiode, for detecting the light beam reflected from the signal recording surface of the optical disk.
In such optical pickup device, so-called focusing servo and tracking servo control operations are performed on the basis of signals detected by the photosensor. By these servo control operations, the light beam projected on the signal recording surface may be converged accurately on the signal recording surface even through the optical disk is subjected during its rotation to so-called plane or core deviation, so that a beam spot formed by the convergence of the light beam will correctly trace the spirally or concentrically extending recording track formed on the signal recording surface.
More specifically, for effecting the focusing servo control operation, a focusing error signal indicating the deviation of the beam spot with respect to the signal recording surface of the optical disk in the direction at right angles with the signal recording surface, that is, along the direction of the optical axis of the light beam, is formed, on the basis of the detection output eignal, as disclosed for example in U.S. Pat. No. 4,059,841. Then, on the basis of this focusing error signal, the optical system for converging the light beam on the signal recording surface, such as an object lens, as disclosed in the above U.S. Pat. No. 4,766,583, is controlled so as to be displaced in a direction towards or away from the signal recording surface.
For effecting the tracking servo control operation, a tracking error signal indicating the deviation of the beam spot from a given recording track along the radial direction of the optical disk is formed on the basis of the detection output from the photosensor and in accordance with, for example, a push-pull method as described in the Japanese Patent Publication No. 3273/1986 or a three-beam method as described in the Japanese Patent Publication No. 13123/1978.
Meanwhile, if the size of the above described optical pickup device is to be reduced, it is necessary to reduce the distance between the semicondutor laser unit and the signal recording surface by reducing the size or the focal distance of the optical device adapted for converging the light beam emanating from the semiconductor laser unit on the signal recording surface of the optical disk.
However, when the distance between the semiconductor laser unit and the signal recording surface is reduced, so-called return light to the semiconductor laser unit increases. The return light means the light beam fraction which is reflected by the signal recording surface and returned to the semiconductor laser unit. In general, an increase in the return light results in increase in the laser unit noise or the noise in light intensities to give rise to various inconveniences, such as fluctuations in the light emitting power, such that it may become occasionally impossible to perform correct write and/or read operations for data signals.
Meanwhile, the semiconductor laser unit may be classified into a so-called refractivity index waveguide type unit oscillating in a single mode and a so-called gain waveguide type unit oscillating in a multiple mode. The gain waveguide type laser unit is subject to the laser unit noise due to return light to a lesser extent than the refractivity index waveguide type unit. Hence, for reducing the size of the optical pickup device, it is more advisable to use the gain waveguide type laser unit than the refractive index waveguide type unit as the light suorce.
However, the gain waveguide type laser unit exhibits a larger astigmatic difference than the refractivity index waveguide type laser unit. The light beam emitted by the semiconductor laser unit is subject to astigmatism due to the astigmatic difference proper to the semiconductor laser unit. The result is that the beam spot formed on the signal recording surface assumes the shape of an ellipsis having its long axis extending in mutually perpendicular directions depending on the de-focusing direction, as shown in FIG. 1.
Referring to FIG. 2, when the beam spot shape is changed as described above on occurrence of de-focusing with resulting fluctuations in the beam spot area measure on the recording track, the maximum level positions of the tracking error signal and the RF signal are not coincident with the focusing position corresponding to the minimum jitter of the read-out or written data signals. That is, the tracking error signal and RF signal levels exhibit non-symmetricity with respect to the focusing position.
As a result of such non-symmetricity of the tracking error signal and RF signal levels with respect to the focusing position, it may become occasionally impossible to effect focusing or tracking servo operations on occurrence of de-focusing due to plane or core deviations of the optical disk.
Conventionally, such astigmatism is corrected by a plane parallel glass which is inclined a predetermined angle with respect to the optical axis of the light beam and which is placed on a light path along which the light beam is dispersed or converged. However, not only it is not possible with the use of the plane parallel correction plate to correct the astigmatism completely, but the use of such correction plate impedes the reduction in size of the optical pickup device.